Putting this type of prosthesis into place poses a problem for surgeons, since the size of the prosthesis to be used, in particular its width, depends on the size of the bone which is to receive it, and in particular on the width of the medullary canal in the bone.
It is therefore necessary for operating theaters to permanently maintain a large stock of prosthesis in a range of graduated widths. Holding and managing such a stock of expensive parts is costly, particularly since they must be stored under sterile conditions. This represents a considerable financial burden for a hospital. In addition, it is not very convenient for a surgeon to have to select the most appropriate prosthesis from such a stock whenever a prosthesis is to be used.
European Published Patent Application No. 85 147 has already proposed a hip joint prosthesis for implanting in the proximal end of the femur, which prosthesis is made up of two independent parts, namely, a main body or spatula, and a wedge-shaped side part. The side part is intended to slide in a guide profile provided on the narrow side face of the spatula.
This prior prosthesis would apparently solve the above-specified problem: in particular, the reference proposes providing a series of side parts of different thicknesses, each of which side parts may be combined with a common spatula in order to cover a range of prosthesis widths. This arrangement is essentially intended to enable the prosthesis to be placed at a specified height in the end of the femur. To this end the prosthesis is implanted in two stages:
(a) firstly, the spatula is placed in medullary canal in the desired final position; and
(b) secondly, the side piece is placed against the spatula and is thrust into the femur while the spatula is held in the desired position until the outer wedge-shaped edge of the side piece comes into contact against bone tissue, and thereby locks the assembly in position by virtue of the wedging effect.
In this prior arrangement, the main body of the prosthesis and the wedge-shaped side part are thus two initially separate items which are brought together solely during prosthesis implantation. These items are never genuinely fixed relative to each other, and as a result they are difficult for the surgeon to handle both before and during implantation, and may give rise to inaccuracies. In addition, after implantation, when forces are applied to the prosthesis (and these forces may be very high, and in various different directions), there is a risk of the two parts working loose relative to each other, and thus of the prosthesis assembly working loose from the bone.
Preferred embodiments of the present invention remedy these drawbacks by providing a prosthesis of the above-described type in which handling is facilitated, both before and during implantation, and which is securely and durably retained by the bone after implantation.